Use a direct include of strings headers in base/.

base/nullable_string16.h

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_NULLABLE_STRING16_H_
 #define BASE_NULLABLE_STRING16_H_
 
-#include "base/string16.h"
+#include "base/strings/string16.h"
 
 // This class is a simple wrapper for string16 which also contains a null
 // state.  This should be used only where the difference between null and
 // empty is meaningful.
 class NullableString16 {
  public:
   NullableString16() : is_null_(false) { }
   explicit NullableString16(bool is_null) : is_null_(is_null) { }
   NullableString16(const string16& string, bool is_null)
       : string_(string), is_null_(is_null) {
   }
 
   const string16& string() const { return string_; }
   bool is_null() const { return is_null_; }
 
  private:
   string16 string_;
   bool is_null_;
 };
 
 #endif  // BASE_NULLABLE_STRING16_H_